A plastic molded type semiconductor device is fabricated by mounting a semiconductor chip on a chip mounting surface of a die pad (which is also called tab) supported on the frame body of a lead frame through a supporting lead, electrically connecting an external terminal, which is disposed on the principal surface of the semiconductor chip, with an inner portion of the lead supported onto the frame body of the lead frame through a bonding wire, sealing the semiconductor chip, die pad, supporting lead, inner portion of the lead, bonding wire and the like with a plastic mold, cutting the supporting lead and the outer portion of the lead from the frame body of the lead frame, and then forming the outer portion of the lead into a predetermined shape.
The plastic mold for the above-described plastic molded type semiconductor device is fabricated in accordance with the transfer mold method suited for mass production. Described specifically, a lead frame subjected to the preceding steps (die bonding step and wire bonding step) is disposed between the top portion and the bottom portion of a mold and at the same time, within the cavity of the mold, a semiconductor chip, die pad, supporting leads and inner portions of the leads and bonding wires are arranged. Then a resin is poured under pressure into the cavity from a pot of the mold through its runner and gate, whereby a plastic mold can be prepared.
In the fabrication step of the above-described plastic mold, with a view to suppressing a failure to fill the resin into the cavity, in other words, suppressing the generation of a void, there has been an attempt to make the fluidity of the resin flowing into the filling region 11A on the principal surface side of the semiconductor chip 2 equal to the fluidity of the resin flowing into the filling region 11B on the reverse surface side of the semiconductor chip 2 by arranging the semiconductor chip 2 and the die pad 3A within the cavity 11 so as to, as illustrated in FIG. 16 (a schematic cross-sectional view), make a clearance L1 from the principal surface of the semiconductor chip 2 to the inside wall surface of the cavity 11 opposite to the principal surface equal to a clearance L2 from the reverse surface of the die pad 3A to the inside wall surface of the cavity 11 opposite to the reverse surface. In addition, there has also been an attempt to fill the resin simultaneously into the filling region 11A on the principal surface side of the semiconductor chip 2 and the filling region 11B on the reverse surface side by adopting, as a gate for controlling the amount of the resin poured into the cavity 2, a center gate 12 (which will also be called "vertical gate") extending above and below the lead frame 3.
In the above-described plastic molded semiconductor device, the die pad, together with the semiconductor chip, is sealed with the plastic mold so that water contained in the plastic mold tends to be collected in the reverse surface of the die pad. Water content collected in the reverse surface of the die pad vaporizes and expands by the heat generated upon a temperature cycle test, which is an environmental test effected after the completion of the product, or the heat generated upon packaging and becomes a cause for cracks (package cracks) of the plastic mold.
With a view to overcoming such technical problems, disclosed in Japanese Patent Laid-Open No. SHO 63-204753 is a technique for making the area of a die pad smaller than that of a semiconductor chip, by which technique, a phenomenon that water contained in the resin of a plastic mold is collected in the reverse surface of the die pad can be suppressed. Thus, cracks (package cracks) of the plastic mold caused by vaporization and expansion of the water content in the reverse surface of the die pad can be prevented.
As illustrated in FIG. 17 (a schematic cross-sectional view), when the area of a die pad 3A is made smaller than that of a semiconductor chip 2, a filling region 11B on the reverse surface side of the semiconductor chip 2 becomes wider in proportion, which makes the fluidity of a resin flowing in the filling region 11B on the reverse surface side of the semiconductor chip 2 higher than that flowing in the filling region 11A on the principal surface side of the semiconductor chip 2. In other words, the filling of the resin into the filling region 11B on the reverse surface side of the semiconductor chip 2 is completed earlier than that into the filling region 11A on the principal surface side of the semiconductor chip 2. As illustrated in FIG. 18 (a schematic cross-sectional view), a resin 1A filled in the filling region 11B on the reverse surface side of the semiconductor chip 2 lifts the semiconductor chip 2 upwardly and causes inconvenient appearance of the semiconductor chip 2, bonding wire and the like from the plastic mold, leading to a marked reduction in the yield of the plastic molded type semiconductor device.
In the resin molded type semiconductor device adopting a QFP structure, on the other hand, supporting leads are arranged in the outside region of the corner of a semiconductor chip, while a plurality of leads and a plurality of bonding wires are arranged on the outside region of each side of the semiconductor chip. In other words, the outside region of the corner of the semiconductor chip is coarser than the outside region of each side of the semiconductor chip so that the fluidity of the resin is higher in the outside region of the corner of the semiconductor chip than the outside region of each side of the semiconductor chip. Accordingly, the bonding wire tends to flow owing to the resin flowing into the outside region of each side of the semiconductor chip from the outside region of the corner and a short circuit occurs between two adjacent bonding wires, which brings about a marked deterioration in a yield of the plastic molded type semiconductor device. The short circuit between these bonding wires is particularly marked between a bonding wire connected to a first-stage lead most adjacent to the outside region of the corner of the semiconductor chip and a bonding wire connected to a second-stage lead adjacent to the first lead.
An object of the present invention is to provide a technique which can increase an yield of a plastic molded type semiconductor device.
Another object of the present invention is to provide a technique which can heighten an yield in the fabrication process of a plastic molded type semiconductor device.
The above-described and the other objects and novel features of the present invention will be apparent by the following description and accompanying drawings.